This is what happened to my HHKB after 5 years of use. The series of photo I am about to post gets even better because I compare my old HHKB with like-new HHKB. You can see how Topre PBT keys wear. Please note that I type lightly and I rarely sweat, both of which I think would negatively affect wear rate.
I didn't clean the board(just some air) before taking pictures, so part of that shine may come off after cleaning.
Side by side photo (from the front.)
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Photo of my 5-year old HHKB at an angle that reflection is most prominent.
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Photo of a like-new HHKB at the same angle
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1:1 crop of the above photo of old HHKB
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1:1 crop of the above photo of like-new HHKB
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The photos are mine, and was posted before at
http://www.kbdmania.net/xe/3510267Edit: I re-read your post and I see that you were asking for pictures of white caps. It is hard to see how textures of key caps wear if they are white. So, I think you could at least see how textures of the Topre PBT keys wear.